U.S. Pat. No. 5,268,086 discloses an air/fuel control system responsive to feedback signals derived from exhaust gas oxygen sensors positioned both upstream and downstream of a catalytic converter. Each sensor has a catalytic electrode positioned in the engine exhaust. During a converter efficiency test cycle, air/fuel control proceeds with a third exhaust gas oxygen sensor having a non-catalytic electrode positioned in the engine exhaust downstream of the catalytic converter substituted for the catalytic sensor. Output signals are then compared for operation in which the catalytic sensor is utilized for feedback operation and the non-catalytic sensor is utilized to provide an indication of degraded converter efficiency.
The inventors herein have recognized numerous problems with the above approaches. One problem is that during the test cycle, when the non-catalytic sensor is substituted in the air/fuel control feedback loop, the average air/fuel ratio may operate lean of stoichiometry, resulting in increased emissions. Such operation occurs because the non-catalytic sensor output will shift towards leaner air/fuel ratios when exhaust gases are not fully equilibrated such as may occur with aging or other degradation of the converter.